Control valves (e.g., sliding stem valves, rotary valves, etc.) are commonly used in process control systems to control the flow of process fluids. Sliding stem valves such as, for example, gate valves, globe valves, etc., typically have a valve stem (e.g., a sliding stem) that moves a flow control member (e.g., a valve plug) disposed in a fluid path between an open position to allow fluid flow through the valve and a closed position to prevent fluid flow through the valve. A control valve typically includes an actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.) to automate the control valve. In operation, a control unit (e.g. a positioner) supplies a control fluid (e.g., air) to the actuator to position the flow control member to a desired position to regulate the flow of fluid through the valve. The actuator may move the flow control member through a complete stroke length between a fully closed position to prevent fluid flow through the valve and a fully open position to allow fluid flow through the valve.
In practice, many process control applications require actuators (e.g., valve actuators) to include fail-safe systems. A fail-safe system provides protection to a process control system by typically causing the actuator and, thus, the flow control member to move to either a fully closed or a fully opened position during emergency situations, power failures, and/or if the control fluid (e.g., air) supply to an actuator (e.g., a pneumatic actuator) is shut down.
Some known piston actuators (e.g. spring-return actuators) may provide a mechanical fail-safe return. For example, these known piston actuators may use an internal spring in direct contact with a piston to provide a mechanical fail-safe return to bias the piston to one end of the stroke travel or the other (e.g. fully opened or fully closed) when the control fluid supply to the actuator fails. However, when used with long-stroke applications (e.g., stroke lengths of four (4) inches or more), such long-stroke spring-return actuators often provide poor control. That is, in certain applications, the spring rate of the bias or fail-safe spring may be sufficient to degrade actuator performance because the supply fluid and the control member must overcome the bias force of the fail-safe spring. Alternately, long-stroke actuators require a spring having a smaller spring rate to accommodate the long-stroke length (i.e., so that the spring can compress the length of the stroke). However, in long-stroke actuators, a spring having a smaller spring rate often lacks sufficient thrust or force to cause the flow control member to sealingly engage a valve seat to prevent leakage through the valve upon a system failure, thereby providing an inadequate fail-safe system.
Double-acting actuators may be used for larger valves requiring long-stroke lengths. Double-acting actuators often provide more accuracy than single-acting actuators because double-acting actuators operate based on a controlled pressure differential across the actuation member (e.g., a piston) and, thus, do not rely on a spring (e.g., a spring rate) to return the actuator to a desired position (e.g., a fully closed position, a fully open position, etc.). However, such known double-acting actuators lack a mechanical fail-safe as provided by the above-noted known spring-return actuators and, thus, are undesirable in some applications.
Many known double-acting actuators use an air-based (e.g., pneumatic) fail-safe system to provide a fail-safe mechanism. However, such known air-based fail-safe systems require additional components (e.g., volume tanks, trip valves/switching valves, volume boosters, etc.), thereby significantly increasing complexity and manufacturing costs. In other examples, some known double-acting long-stroke actuators include a bias or fail spring fail-safe system that directly and continuously acts upon the actuator (e.g., the piston) during operation. However, such bias or fail spring approaches require an oversized piston to overcome the spring forces of the bias or fail spring.